The present invention relates to a supermagnetostrictive alloy or a magnetic alloy capable of exhibiting a giant magnetostriction through a phase transformation, and a method for preparation thereof.
A functional material usable as a member for generating a shift (magnetostriction) and a force (stress) is referred to as actuator, which is used in a wide range of fields such as electronic devices, medical instruments and production apparatuses. Such a functional material includes a piezoelectric material, a magnetostrictive material, a shape-memory material and others. One of notable magnetostrictive materials is a rare-earth alloy Tb0.3Dy0.7Fe (Journal of Alloys and Compounds, vol. 258, 1997), which has been commercialized (Trade Name: Terfenol-D). Terfenol-D has a maximum magnetostriction of 0.17%.
Japanese Patent Laid-Open Publication No. 11-269611 discloses a Fe/Pt-based or Fe/Pd-based rapidly solidified alloy having a magnetostriction of 0.15 to 0.2%. Further, Ni2MnGa is known as a shape-memory alloy whose crystal state is changed by magnetic field or electric field to provide a high-speed activation (Industrial Materials Vol. 45, No. 12, Nov. 1997, pp 108-111, Japanese Patent Laid-Open Publication No. 10-259438). However, this shape-memory alloy has mechanical brittleness and poor workability. Japanese Patent Laid-Open Publication No. 62-170453 discloses another shape-memory alloy which contains 25-30 at % of Pt added into Fe and has an irregular atomic arrangement to provide enhanced workability. Japanese domestic publication of PCT application in Japanese language No. 11-509368 discloses a method for controlling a material having a twin structure to cause change in shape and generate movement and/or force in the material by applying to the material a magnetic field having a directionality and magnitude suitable for achieving a desired reorientation of the twin structure.
In line with advances in downsizing of electronic devices and upgrading of medical instruments and production apparatuses, it is desired to achieve an actuator made of a material capable of providing a larger shift (lager magnetostriction) with excellent workability.
With focusing on a phase transformation in an alloy structure, the inventors has successively achieved a magnetostriction of 0.3% or more, particularly 0.5% or more by subjecting Fe3xe2x88x92xPt1+x(xe2x88x920.02xe2x89xa6xc3x97xe2x89xa60.2) to a heat treatment under inventive conditions for ordering.
Specifically, the present invention is directed to a supermagnetostrictive alloy having a degree of order of 0.6 to 0.95 achieved by subjecting Fe3xe2x88x92xPt1+x(xe2x88x920.02xe2x89xa6xc3x97xe2x89xa60.2) to a heat treatment. Even if the ordering is performed to a Fe3xe2x88x92xPt1+x alloy having less than xe2x88x920.02 or greater than 0.2 of x, no FCC-FCT martensitic transformation is caused. A preferable range of x is 0.0xe2x89xa6xc3x97xe2x89xa60.1.
Further, the present invention is directed to a method for the preparation of the above supermagnetostrictive alloy comprising the steps of subjecting a Fe3xe2x88x92xPt1+x(xe2x88x920.02xe2x89xa6xc3x97xe2x89xa60.2) alloy of a raw material to a homogenization annealing, and then subjecting the resulting product to a heat treatment at 700 to 1000 K for 0.5 to 600 hours. Given that when all of Pt and Fe in Fe3Pt having a face-centered structure are accurately arranged at corners and face-centers of the crystal structure, respectively, such a state is defined as a fully ordered state or the degree of order S=1, a maximum magnetostriction could be achieved by arranging the degree of order in the range of 0.6 to 0.95. When the degree of order is less than 0.6 or greater than 0.95, no FCC-FCT martensitic transformation is caused.
The alloy of the present invention exhibits an extremely large magnetostriction of 0.5% even under a weak magnetic field of about 4 T. As for magnetostrictive alloys, by extension of a conventional conception in which spins are coordinated with each other in a magnetic domain, an obtainable magnetostriction xcex94 I/I is the order of 10xe2x88x926 at utmost. One of the main reasons for the extremely large magnetostriction of 0.5% (5xc3x9710xe2x88x923) in the present invention is that a crystallographic domain (variant) is conformed to a magnetic domain to form a single domain and thereby spin axes are crystallographically coordinated with each other as well.
The present invention provides an alloy having excellent workability and thereby the alloy can be formed in a single crystal bulk, polycrystal bulk, thin sheet shape (including roll shape), linear shape, thin film shape or the like. The present invention can also provide a shape-memory alloy in which the crystallographic domain (variant) and magnetic domain are approximately equalized in size and aligned in the direction of an applied magnetic field.